The long-term goal of this project in renal physiology is to elucidate the intrarenal mechanisms that regulate nephron function. The studies proposed in this continuation application will evaluate three hypotheses related to 1) tubuloglomerular feedback (TGF) signal transmission across the macula densa in the juxtaglomerular apparatus (JGA), 2) the initiation and propagation of the TGF signal across the extraglomerular mesangium to the afferent arteriole, and 3) synergistic interactions between TGF and myogenic autoregulatory mechanisms. These studies will be performed directly in preglomerular vessels and the macula densa region of in vitro, blood-perfused, juxtamedullary nephrons and cultured mesangial cells from rat kidney. The specific aims are: 1. We will evaluate the transport-coupling hypotheses of TGF signal transmission with micropuncture and electrophysiological studies to determine a) the transepithelial potential differences at the macula densa and during a TGF response; b) the net water flux and electrical current through the macula densa cell plaque; c) whether solute diffusion into the JGA interstitium is limited in comparison to the cortical peritubular interstitium. 2. We will determine if local changes in extracellular ion concentration and/or osmolarity, which are thought to be generated during a TGF response, stimulate cultured mesangial cells, ad if the cytosolic calcium response is propagated to neighboring cells. 3. We will determine if late afferent vasoconstriction induces synergistic myogenic contraction of upstream vascular segments via elevated intravascular pressure, and the impact of this mechanism on the autoregulatory responses of proximal afferent and interlobular arteries. The TGF myogenic autoregulatory mechanisms are important mediators of kidney function, and they are known to be reset or inhibited in a number of pathophysiological states. The results of this basic research into the mechanisms will be relevant to a number of serious and costly renal diseases, including diabetic nephropathy, chronic renal failure, and hypertension.